1. Field of the Invention
The present invention relates to a shell main body for a muffler that is used in an exhaust system of a combustion engine mounted on a motor vehicle or the like and is equipped with an inner shell and an outer shell wrapping an outer surface of the inner shell.
2. Description of the Related Art
As a shell main body for a muffler with an inner shell and an outer shell, a shell main body disclosed in Japanese Patent Application Laid-open No. 2002-206422 is conventionally known. As shown in a schematic cross-sectional view in FIG. 4, a shell main body 103 of this conventional muffler is formed of two layers of an inner shell 101 and an outer shell 102. Each of the inner and outer shells 101 and 102 has a cylindrical shape with a cross section that is not a perfect circle but is defined by small arc portions 104 and 104 alternating with large arc portions 105 and 105, similarly to an oval, a racing track-like figure, or the like.
The conventional shell main body 103 for the muffler described above, however, has the following problem. The shell main body 103 is provided at its edge portions with openings, which are closed by not-shown end plates by press caulking or welding, respectively. This structure makes it impossible for the end plates and the inner shell 101 to be welded to each other, thus causing not-shown minute gaps which are located between an inner surface of the end plates and the edge portions of the inner shell 101 and communicate an inner space of the shell main body 103 and gaps 104a, 104b, 105a, and 105b formed between the inner shell 101 and the outer shell 102, because the inner shell 101 are covered by the end plates and the outer shell 102.
Incidentally, the edge portions of the inner shell 101 and the outer shell 102 have overlap portions 108 that are welded with each other in a state that they are overlapped with each other at a top position and in a circumstantial direction of the shell main body 103.
Consequently, water enters the gaps 104a, 104b, 105a, and 105b formed between the inner shell 101 and the outer shell 102 due to capillary phenomenon through the gaps between the end plates and the inner shell 101 and accumulates on an inner portion of the muffler when water vapor in exhaust gas is cooled off into the water due to contact with a low-temperature shell wall of the muffler or drop in temperature after stopping an engine.
Then, the water that has entered the gaps 104a, 104b, 105a, and 105b is heated by high-temperature exhaust gas passing through the muffle to vaporize after restarting the engine, resulting in pressure rise of the vapor in the gaps 104a, 104b, 105a, and 105b. When this pressure rise speed exceeds a discharging speed of the vapor that is discharged from the gaps 104a, 104b, 105a, and 105b to the inner portion or an outside of the shell main body 103, vapor pressure in the gaps 104a, 104b, 105a, and 105b between the shells 101 and 102 sometimes rises to deform them.
Incidentally, water causing such a problem sometimes splashes onto an outer periphery of the shell main body 103 and enters into the gaps between the inner shell 101 and the outer shell 102.
Such a problem can be solved if a communicating path, for example a hole, is formed on a shell overlap portion at an upper surface side of the shell main body 103 so as to allow the gap between the inner shell 101 and the outer shell 102 to communicate with the outside of the outer shell 102 or gaps at the edge portion of the outer shell 102 is formed so that the vapor entered into the gap can pass through the gaps of the outer shell 102.
However, in a shell main body 103 having a cylindrical shape with a cross section that is not a perfect circle but is defined by small arc portions 104 and 104 alternating with large arc portions 105 and 105, similarly to an oval, a racing track, or the like, the inner shell 101 and the outer shell 102 in the right and left small arc portions 104 and 104 are in close contact with each other, clogging their right and left side gaps 104a and 104b and dividing the gap into an upper gap 105b and a lower gap 105a in the large arc portions 105 and 105 as shown in FIG. 5.
This clogging prevents the vapor generated in the lower gap 105a between the lower portions of the inner shell 101 and the outer shell 103 from discharging from the lower gap 105a, resulting in pressure-rise of the vapor to apply its pressure force P to and deform the lower portions of the shells 101 and 102 in an expansion direction. The lower large arc portion 105 of the shells 101 and 102 is deformable easily more than the small arc portions 104 and 104, because the former has stiffness smaller than the former.
It is undesirable to provide a lower side of the outer shell 102 with a not-shown communicating path or a not-shown shell overlap portion, similar to the shell overlap portion 108, that is fluidically connectable the lower gap 105a to the outside of the outer shell 102, because a splash and/or mud is easily allowed to enter the lower gap 105a through the communicating path or the shell overlap portion and rust the shell main body 103. Consequently, vapor entered the lower gap 105a can escape to nowhere, resulting in pressure rise in the lower gap 105a between the inner shell 101 and the outer shell 102. This may possibly deform the inner shell 101 and the outer shell 102.